Context. Massive stars play an important role in shaping the structure of galaxies. Infrared dark clouds (IRDCs), with their low temperatures and high densities, have been identified as the potential birthplaces of massive stars. In order to understand the formation processes of massive stars the physical and chemical conditions in infrared dark clouds have to be characterized. Aims. The goal of this paper is to investigate the chemical composition of a sample of southern infrared dark clouds. One important aspect of the observations is to check, if the molecular abuncances in IRDCs are similar to the low-mass pre-stellar cores, or whether they show signatures of more evolved evolutionary stages. Methods. We performed observations toward 15 IRDCs in the frequency range between 86 and 93 GHz using the 22-m Mopra radio telescope. In total, 13 molecular species comprising N 2 H + , 13 CS, CH 3 CN, HC 3 N, HNC, HCO + , HCN, HNCO, C 2 H, SiO, H 13 CO + , H 13 CN and CH 3 C 2 H were observed for all targets. Hence, we included in general species appropriate for elevated densities, where some of them trace the more quiescent gas while others are sensitive to more dynamical processes. Results. We detect HNC, HCO+ and HNC emission in all clouds and N 2 H + in all IRDCs except one. In some clouds we detect SiO emission. Complicated shapes of the HCO + emission line profile are found in all IRDCs. Both signatures indicates the presence of infall and outflow motions and beginning of star formation activity, at least in some parts of the IRDCs. Where possible, we calculate molecular abundances and make a comparison with previously obtained values for low-mass pre-stellar cores and high-mass protostellar objects (HMPOs). We show a tendency for IRDCs to have molecular abundances similar to low-mass pre-stellar cores rather than to HMPOs abundances on the scale of our single-dish observations.

Abstract. We present the results of N 2 H + (1-0) observations of 35 dense molecular cloud cores from the northern and southern hemispheres where massive stars and star clusters are formed. Line emission has been detected in 33 sources, for 28 sources detailed maps have been obtained. Peak N 2 H + column densities lie in the range: 3.6 × 10 12 −1.5 × 10 14 cm −2 . Intensity ratios of (01-12) to (23-12) hyperfine components are slightly higher than the LTE value. The optical depth of (23-12) component toward peak intensity positions of 10 sources is ∼0.2−1. In many cases the cores have elongated or more complex structures with several emission peaks. In total, 47 clumps have been revealed in 26 sources. Their sizes lie in the range 0.3-2.1 pc, the range of virial masses is ∼30−3000 M . Mean N 2 H + abundance for 36 clumps is 5 × 10 −10 . Integrated intensity maps with axial ratios <2 have been fitted with a power-law radial distribution ∼r −p convolved with the telescope beam. Mean power-law index for 25 clumps is close to 1.3. For reduced maps where positions of low intensity are rejected mean power-law index is close to unity corresponding to the ∼r −2 density profile provided N 2 H + excitation conditions do not vary inside these regions. In those cases where we have relatively extensive and high quality maps, line widths of the cores either decrease or stay constant with distance from the center, implying an enhanced dynamical activity in the center. There is a correlation between total velocity gradient direction and elongation angle of the cores. However, the ratio of rotational to gravitational energy is too low (4× 10 −4 -7.1 × 10 −2 ) for rotation to play a significant role in the dynamics of the cores. A correlation between mean line widths and sizes of clumps has been found. A comparison with physical parameters of low-mass cores is given.

Abstract. We have conducted a survey of SiO emission towards galactic H 2 O and OH masers and ultracompact HII regions using the 15-m SEST and the 20-m Onsala telescope. With the SEST the transitions (v = 0, J = 2 − 1) and (v = 0, J = 3−2) of SiO at 3 and 2 mm were measured simultaneously. With Onsala only the (v = 0, J = 2 − 1) line was accessible. Altogether 369 objects were observed and SiO was detected towards 137 of them. The detection rate is highest towards the most intense H 2 O masers, which probably require powerful shocks to be excited. The SiO detection rate correlates also with the integrated farinfrared flux density and the FIR luminosity of the associated IRAS point source, indicating that the occurrence of shocks is related to the amount of radiation from the central stellar source(s). For flux and luminosity limited samples the SiO detection rate is higher in the inner 7 kpc from the galactic centre than elsewhere. This suggests that dense cores belonging to the so called "molecular ring" provide particularly favourable conditions for the production of gaseous SiO.The full widths above 2σ of the SiO(J = 2 − 1) lines, which are likely to be related to the associated shock velocities, range from 2 to 60 km s −1 except for the line in Ori KL which has a full width of about 100 km s −1 . The median of our sample is 19 km s −1 . The SiO lines are singlepeaked and the peak velocities are always close to the ambient cloud velocity as determined from published CS observations. These line characteristics are compared with the predictions of kinematical bow-shock models. The SiO line shapes correspond with the model of Raga & Cabrit (1993) where the emission arises from turbulent wakes behind bow-shocks. However, the number of symmetric, relSend offprint requests to: J. HarjuThe entire Tables B.1 and B.2 and the spectra of all the detected SiO sources are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html atively narrow profiles indicates that at least in some of the observed sources SiO emission arises also from the quiescent gas component.We suggest that this is due to evaporation of silicon compounds from grain mantles and their reprocessing to SiO in dense quiescent gas according to the model of McKay (1995McKay ( , 1996. These reactions may be initiated and sustained by ionizing radiation from shocks, in the same way as the enhancement of HCO + near Herbig-Haro objects has been explained in the model of Wolfire & Königl (1993).The excitation temperatures of SiO(J = 2 − 1) and (J = 3 − 2) transitions were determined towards three strong sources using measurements in isotopically substituted SiO. In all three sources the transitions are clearly subthermally excited, implying moderate densities (< 3 10 6 cm −3 ) in the SiO emission regions.

We report the results of our observations of the S255IR area with the SMA at 1.3 mm in the very extended configuration and at 0.8 mm in the compact configuration as well as with the IRAM-30m at 0.8 mm. The best achieved angular resolution is about 0.4 arcsec. The dust continuum emission and several tens of molecular spectral lines are observed. The majority of the lines is detected only towards the S255IR-SMA1 clump, which represents a rotating structure (probably disk) around the young massive star. The achieved angular resolution is still insufficient for conclusions about Keplerian or non-Keplerian character of the rotation. The temperature of the molecular gas reaches 130-180 K. The size of the clump is about 500 AU. The clump is strongly fragmented as follows from the low beam filling factor. The mass of the hot gas is significantly lower than the mass of the central star. A strong DCN emission near the center of the hot core most probably indicates a presence of a relatively cold ( 80 K) and rather massive clump there. High velocity emission is observed in the CO line as well as in lines of high density tracers HCN, HCO + , CS and other molecules. The outflow morphology obtained from combination of the SMA and IRAM-30m data is significantly different from that derived from the SMA data alone. The CO emission detected with the SMA traces only one boundary of the outflow. The outflow is most probably driven by jet bow shocks created by episodic ejections from the center. We detected a dense high velocity clump associated apparently with one of the bow shocks. The outflow strongly affects the chemical composition of the surrounding medium.